Prior art pallet conveyors generally have two different modes of operation: synchronous and asynchronous systems. In known synchronous pallet conveyors, the conveyor is intermittently driven to intermittently move pallets between workstations. Synchronous pallet conveyors may be relatively simple to make and use since the pallets are typically frictionally conveyed, no stop mechanism is typically used, and pallets are generally not disengaged or removed from the conveyor at the workstations. A disadvantage of synchronous systems is that the throughput speed of the conveyor line is limited to the throughput speed of the slowest workstation. Another disadvantage is that the frequent starting and stopping may unduly wear the conveyor drive mechanism.
Asynchronous pallet conveyors, which are also known, reduce the wear associated with intermittent use of the conveyor drive mechanism by providing a continuously driven conveyor. U.S. Pat. No. 3,088,197 to Cargill, U.S. Pat. No. 3,666,076 to Miller, and U.S. Pat. No. 3,934,701 to Mooney all disclose asynchronous pallet conveyors.
In Cargill, a clutch mechanism freewheels a pinion permitting accumulation or stopping of pallets conveyed between workstations by a continuously moving conveyor.
Miller provides a boss on the pallets for engaging notches on a continuous conveyor band. A cam surface disengages the boss for accumulation or stopping at a workstation.
Mooney discloses an indexing machine in which pallets have a cam follower which is received in a path to release a latch holding the pallet to the conveyor and thus removing the pallet from the conveyor at the workstation. Mooney also provides for deceleration of pallets upon removal from the conveyor and acceleration of pallets in order to reduce wear associated with the latch which holds the pallet to the continuously moving conveyor.
U.S. Pat. No. 3,805,942 to Auerhammer discloses a selective synchronous, nonsynchronous transfer machine having an intermittently driven conveyor including subpallets carried thereon, and workpiece pallets releasably latchable to the subpallets for conveyance between workstations.
Although asynchronous systems may improve overall throughput speed of conveyor lines by enabling accumulation of pallets at each workstation, accumulation may disadvantageously require relatively complicated mechanisms which either remove or disengage pallets from the continuously driven conveyors. Moreover, most of the engaging mechanisms are activated on both the horizontal and sloped sections of the conveyor.
What is desired, therefore, is a pallet conveyor which permits accumulation of pallets without use of a complicated mechanism for disengaging or removing the pallet from the conveyor, and which also enables pallets to be transported along sloped sections of a conveyor line by engaging the conveyor. It is further desirable that the pallet be moving at substantially the same speed as the conveyor when the pallet engages the conveyor to reduce wear on the engaging mechanism.